1. Field of the Invention
The invention relates to methods of pressure welding and has specific reference to friction welding.
The present invention may be of utility in mechanical and electrical engineering, tool manufacture and in the chemical industry.
2. Description of the Prior Art
The invention holds out special promise in friction welding of materials with distinctly different mechanical and thermal properties and those which tend to form scale at their end faces. Friction welding under constant axial pressure is inapplicable in the above cases due to non-uniform distribution of the temperature pattern over the cross section of the parts welded, which impairs weld quality. The known methods of friction welding under a pulsatory axial pressure provide for a better distribution of the temperature pattern over the heat-affected zone. However, no adequate uniformity in the distribution of the temperature pattern can be obtained in welding materials with distinctly different mechanical and thermal properties. Poor penetration at the centre and periphery of the weld is likely to occur in this case.
Known in the art is a method of friction welding (cf. V.I. Vill "Friction Welding of Metals", Leningrad, Mashinostroenie, 1970, p. 61 wich is realized practically in all friction welding machines. It consists in jigging coaxially the parts to be welded, setting them to rotate with respect to each other under a constant axial pressure and upset-forging the parts together on heating up the weld area adequately.
The known method fails to provide for a uniform redistribution of the temperature pattern over the weld area when dissimilar materials having distinctly different mechanical and thermal properties are being welded. Weld quality and reliability consequently suffer. A high axial pressure applied during the welding may bring about an X-shaped distortion of the heat-affected zone resulting in poor penetration at the centre. A low axial pressure may cause convexity of the heat-affected zone accompanied by poor penetration at the periphery of the weld.
Also known is a method of friction welding (cf. USSR Inventor's Certificate No. 1209398, IPC B23K 20/12, published in 1986) consisting in jigging coaxially the parts to be welded the materials whereof have inherent Young's moduli, setting the parts to rotate with respect to each other under an axial pressure which varies recurrently in order to heat the parts up and upsetforging the parts together on heating them up.
In this method of welding, the axial pressure applied in order to heat up the work changes from a minimum value to a maximum one during every period of a different frequency. Consequently the value of the maximum pressure rises incessantly. This fact widens the field of application of the method, rendering the machines capable of welding the parts with a diameter which is larger than one for which they have been designed. However, a variable frequency with which the axial pressure is recurrently changed fails to provide for a redistribution of the temperature pattern in a way eliminating poor and non-uniform heating up of the weld zone.
A rise in the axial pressure to a maximum value in the course of several applications brings about a thermal effect during the heating up which changes in the direction from the centre to the periphery of the weld in materials with distinctly different mechanical and thermal properties. Poor penetration at the periphery and an unjustified lengthening of the heating up period are likely to occur.